SAN FRANCISCO — It’s one of the hottest topics in cancer research today — the idea that cancer stem cells exist and recreate tumors that have been eradicated by radiation or chemotherapy.

Such cells may be impervious to traditional cancer treatments.

The theory has excited many scientists because if true, it could lead to new ways to attack the root cause of cancer.

“Cancer stem calls have certainly caught the imagination of both the scientific community as well as the public as being a new avenue for thinking about how cancer develops and how it progresses and importantly, how it can be treated,” said Martin McMahon, a cancer biology professor at the University of California, San Francisco.

McMahon helped organize a two-day Stem Cells and Cancer symposium at UC San Francisco last week that brought together more than 700 stem cell scientists, cancer biologists and others to explore the idea.

A cancer stem cell can be compared to a dandelion, said Jeffrey Rosen, professor of molecular and cellular biology at Baylor College of Medicine. People mow their lawns and it looks like the dandelions are gone, but they reemerge a short time later because their roots have not been destroyed.

The existence of cancer stem cells could prompt scientists to adopt a dual approach for attacking cancers — one set of therapies to shrink tumors and another to go after the cancer stem cells that are unaffected by current treatments. That could provide new hope for cancer patients.

“Tumor relapse and tumor metastasis are what kill most of the people in our day,” said Emmanuelle Passegue, an assistant professor of medicine at UC San Francisco who also helped organize the conference. Metastasis refers to cancers that have spread to other parts of the body.

Not everyone is sold on the cancer stem cell theory. Skeptics worry that it will prompt researchers to waste time and resources without proof that such cells exist.

Numerous questions remain. But many of the symposium participants said the idea merits further exploration, particularly as scientists gain a deeper understanding of cancer.

“Normal” stem cells have intrigued Californians for several years because of their ability to renew and repair tissues. Voters here have approved a $3 billion research program in the hope that such cells will lead to therapies or cures for a host of diseases. Embryonic stem cells are considered to have the most promise because of their ability to transform into virtually any cell type in the body.

The idea of a dangerous cancer stem cell may seem new, but several conference speakers cited studies that hint at their existence. They noted that various cells within a tumor behave differently. Some are unaffected by the radiation and chemotherapy that kills the bulk of tumor cells. When such radiation-resistant cells are transplanted into mice, they often will recreate a tumor.

Among the unresolved questions is exactly what a cancer stem cell is, and whether it can be found in all cancer types, or just some.

One theory is that a cancer stem cell is a normal stem cell run amok that produces cancerous cells.

Another idea is that certain cells reacquire stem cell-like properties and suddenly have the ability to produce cancerous cells.

Or it may be a cell’s environment that influences its behavior.

“One possibility is the case of a brain cancer stem cell where the cells are not different, it’s just their location in the tumor,” Passegue said. Cells close to a blood vessel, for example, seem to be protected and are not killed as efficiently as other tumor cells.

“They may act as cancer-initiating stem cells because they are protected and they regrow the tumor,” she said.

There are numerous avenues for scientists to explore. By merging the work of stem cell scientists with that of cancer researchers, they hope to gain a deeper understanding of what causes cancer and develop treatments that prevent its return.

“I think even the skeptics are fascinated to watch where the science takes us,” McMahon said.

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